Seaside photographers have a special fondness for the sunset. On rare occasions they can catch the elusive green flash--a split-second pulse of verdant light that signals the disappearance of the sun beneath the ocean waves. On April 7th, astronomy professor Jimmy Westlake photographed a green flash as he stood atop Hawaii's Mauna Kea volcano. The flash he saw, however, did not come from the ocean waves. It happened in the clouds:

"The SKY Club from Colorado Mountain College got an emerald surprise," he says. "Several students visually witnessed the green flash while I was photographing it."

"This appears to be a cloud-top flash," says atmospheric optics expert Les Cowley. "They are not fully understood but might be produced by an inversion layer overlaying the clouds. We need more observations and associated weather data to understand them better."

Basically, a temperature inversion bends the sun's rays to form a mirage that vertically magnifies the tiny color separation which is always present at the edge of the setting sun. Indeed,in Westlake's flash, we see not only green, but also yellow, red, and even a wisp of blue. "The intensity of the blue-green flash caught everyone by surprise!" says Westlake.www.spaceweather.com